package com.wc.算法基础课.F第六讲贪心.修理牛棚;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;

/**
 * @Author congge
 * @Date 2024/4/12 14:19
 * @description https://www.acwing.com/problem/content/1351/
 */
public class Main {
    static FastReader sc = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);
    static int N = 210;
    static int[] a = new int[N];
    static int[] d = new int[N];
    static int m, s, c;

    public static void main(String[] args) {
        m = sc.nextInt();
        s = sc.nextInt();
        c = sc.nextInt();
        for (int i = 1; i <= c; i++) a[i] = sc.nextInt();
        Arrays.sort(a, 1, c + 1);
        int res = a[c] - a[1] + 1;
        // 这里空隙，第一个前面是没有空隙的
        for (int i = 2; i <= c; i++) d[i] = a[i] - a[i - 1] - 1;
        Arrays.sort(d, 1, c + 1);
        // 需要m - 1块缝隙
        for (int i = c; i > c - m + 1 && i >= 1; i--) {
            res -= d[i];
        }
        out.println(res);
        out.flush();
    }
}

class FastReader {
    StringTokenizer st;
    BufferedReader br;

    FastReader() {
        br = new BufferedReader(new InputStreamReader(System.in));
    }

    String next() {
        while (st == null || !st.hasMoreElements()) {
            try {
                st = new StringTokenizer(br.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.parseInt(next());
    }

    String nextLine() {
        String s = "";
        try {
            s = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return s;
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    // 是否由下一个
    boolean hasNext() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                String line = br.readLine();
                if (line == null)
                    return false;
                st = new StringTokenizer(line);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return true;
    }
}
